The basic element structure of a light-emitting diode (LED) comprises a crystal substrate, and an n-type semiconductor layer, a light-emitting layer (including DH structure, MQW structure, SQW structure) and a p-type semiconductor layer sequentially grown thereon, wherein each of the n-type layer, the conductive crystal substrate (SiC substrate, GaN substrate and the like) and the p-type layer has an external outlet electrode.
For example, FIG. 8 shows one exemplary constitution of an element (GaN group LED) comprising a GaN group semiconductor as a material of a light-emitting layer, wherein a GaN group crystal layer (n-type GaN contact layer (also a clad layer) 102, a GaN group semiconductor light-emitting layer 103 and a p-type GaN contact layer (also a clad layer) 104) are sequentially laminated on a crystal substrate 101 by crystal growth, and a lower electrode (generally an n-type electrode) 105 and an upper electrode (generally a p-type electrode) 106 are set. In this specification, the layers are mounted on a crystal substrate (downside) and the light goes out upward, for the explanation's sake.
In LED, an important issue is how efficiently the light produced in the light-emitting layer can be externally taken out (what is called, light-extraction efficiency). Therefore, various designs have been conventionally tried such as an embodiment wherein an upper electrode 106 in FIG. 8 is rendered a transparent electrode so that the light heading upward from the light-emitting layer will not form an obstacle to the outside, an embodiment wherein the light heading downward from the light-emitting layer is returned upward by forming a reflective layer and the like.
For the light emitted from the light-emitting layer in the vertical direction, the light-extraction efficiency can be improved by making the electrode transparent or forming a reflective layer, as mentioned above. However, of the lights advancing in the spreading direction of the light-emitting layer (direction shown with a thick arrow in a light-emitting layer 103 in FIG. 8, hereinafter to be also referred to as a “lateral direction”), the light that reaches the sidewall within the total reflection angle defined by a refractive index differential can be externally emitted, but many other lights repeat reflection between, for example, sidewalls, are absorbed in an element, particularly by a light-emitting layer itself, attenuated and disappear. Such lights in the lateral direction are enclosed by upper and lower clad layers or a substrate (sapphire substrate) and an upper clad layer, or a substrate and an upper electrode (further, a coating substance on the outside of the element and the like), and propagated in the lateral direction. The light that propagates in the lateral direction occupies a large portion of the entire light amount produced by a light-emitting layer, and in some cases it amounts to 60% of the whole.
With regard to a flip-chip type LED (light goes out through a substrate) to be mounted with the substrate on the upper side, an embodiment is known wherein a side wall of a laminate, which is an element structure, has an angle and the side wall is used as a reflection surface toward the substrate side, so that such light in the lateral direction can be reflected in the substrate direction. However, cutting 4 facets of a small chip with an angle is a difficult processing, posing a problem in costs.
Furthermore, the light advancing in the vertical direction is also associated with problems in that a standing wave that repeats reflection between an interface of GaN group semiconductor layer/sapphire substrate and an interface of GaN group semiconductor layer/p-type electrode (or sealing material) is formed and the like, which in turn hinder light-extraction efficiency.
It is a first object of the present invention to provide a light-emitting element having a novel structure capable of solving the above-mentioned problem, directing the light in the lateral direction, which is produced in the light-emitting layer, to the outside, and further, suppressing the occurrence of the above-mentioned standing wave.
In addition to the problem of the light-extraction efficiency as mentioned above, the following problem of lower output is present when InGaN is used as a material of a light-emitting layer and the ultraviolet light is to be emitted.
A light-emitting element comprising InGaN as a light-emitting layer generally provides highly efficient emission. This is explained to be attributable to a smaller proportion of carriers captured by the non-radiative center, from among the carriers injected into the light-emitting layer, due to the localization of the carriers caused by fluctuation of the In composition, which in turn results in a highly efficient emission.
When a blue purple light—ultraviolet light having a wavelength of not more than 420 nm is to be emitted by a GaN group light-emitting diode (LED) and a GaN group semiconductor laser (LD), InGaN (In composition not more than 0.15) is generally used as a material of a light-emitting layer, and the structure involved in the emission is a single quantum well structure (what is called a DH structure is encompassed because of a thin active layer) or a multiple quantum well structure.
In general terms, the upper limit of the wavelength of the ultraviolet light is shorter than the end (380 nm–400 nm) of the short wavelength of visible light, and the lower limit is considered to be about 1 nm (0.2 nm–2 nm). In this specification, the blue purple light of not more than 420 nm emitted by the above-mentioned InGaN having an In composition of not more than 0.15 is also referred to as an ultraviolet light and a semiconductor light-emitting element emitting such ultraviolet light is referred to as an ultraviolet light-emitting element.
The ultraviolet light GaN can produce has a wavelength of 365 nm. Therefore, in the case of a ternary system wherein InGaN essentially contains In composition and free of Al composition, the lower limit of the wavelength of the ultraviolet light which can be generated is longer than the aforementioned 365 nm.
When compared to blue and green light-emitting elements having a light-emitting layer having a high In composition, the ultraviolet light-emitting element has a shorter wavelength. Thus, the In composition of the light-emitting layer needs to be reduced. As a consequence, the effect of the localization of the aforementioned fluctuation of the In composition decreases and the proportion thereof to be captured in the non-radiative recombination center increases, which prevents a high output. Under the circumstances, dislocation density, which causes the non-radiative recombination center, has been actively reduced. As a method for reducing the dislocation density, ELO method (lateral growth method) can be mentioned, and high output and long life have been achieved by reducing the dislocation density (see reference (Jpn. J. Appl. Phys. 39(2000) pp. L647) etc.).
In a GaN group light-emitting element, a light-emitting layer (well layer) is sandwiched between clad layers (barrier layers) made of a material having a greater band gap. According to a reference (Hiroo Yonezu, Hikari Tsushin Soshi Kogaku, Kougakutosho Ltd., p. 72), a guidance of setting the difference in the band gap to generally not less than “0.3 eV” has been provided.
From the above-mentioned background, when InGaN having a composition capable of emitting ultraviolet light is to be used as a light-emitting layer (well layer), the clad layer (a single quantum well structure contains not only a clad layer but also a barrier layer) used to sandwich the light-emitting layer is AlGaN having a greater band gap in view of enclosure of the carrier.
In addition, when a quantum well structure is to be constituted, the barrier layer needs to have a thickness of a level producing a tunnel effect, which is generally about 3–6 nm.
For example, FIG. 9 shows one embodiment of a conventional light-emitting diode using In0.05Ga0.95N as a material of a light-emitting layer, which has an element structure wherein an n-type GaN contact layer 202, an n-type Al0.1Ga0.9N clad layer 203, an In0.05Ga0.95N well layer (light-emitting layer) 204, a p-type Al0.2Ga0.8N clad layer 205 and a p-type GaN contact layer 206 are sequentially laminated on a crystal substrate S10 via a buffer layer 201, by crystal growth, and a lower electrode (generally an n-type electrode) P10 and an upper electrode (generally p-type electrode) P20 are formed.
However, the ELO method is problematic in that the methods for growing a GaN layer to be a base, forming a mask layer and re-growing are necessary, and growth in a number of times is necessary, thus increasing the number of steps. In addition, because a re-growth interface exists, it has a problem that, although dislocation density reduces, the output does not increase easily.
The present inventors have studied conventional element structures in an attempt to use InGaN as a material of the light-emitting layer and achieve higher output of ultraviolet light, and found that AlGaN layer is behind the distortion relative to the InGaN light-emitting layer, which results from the difference in the lattice constant.
It has been also found that, when a barrier layer is made thin in the quantum well structure, Mg is diffused from the p-type layer formed thereon to a light-emitting layer and forms a non-radiative center, thus problematically preventing high output of an ultraviolet light-emitting element.
A second object of the present invention is to achieve high output, and further, a long life, by optimizing the structure of the element, when InGaN is used as a material of a light-emitting layer of the light-emitting element of the present invention and ultraviolet light is to be emitted.